And that's it - never goes farther. It's a headless machine, but I never hear any disk activity, and cannot ping, and even sending a break on the serial console doesn't get me back to the OBP prompt - it's hung hard.
I copied the gentoo-2.4 kernel from the cd, and it boots fine - but I cannot get it to execute the kernels I build. I tried compiling it with egcs64, but that just succeeded in setting me back a day while I reinstalled gcc-sparc64. I was thinking that my kernel wasn't being compiled as 64 bit, but it is:

I remember checking my kernel size, and it was just under the 3.5 MB limit - but in my messing around trying to get it going it seems to have grown to 3.7 MB! Fixed that - made it smaller, but same problem still exists. Looks like you saved me from a problem I hadn't gotten far enough yet to encounter! Thanks.